In an electrophotographic apparatus, such as a copying machine, an exposing lamp and a fusing lamp are typical power consuming members connected to the same power source in parallel.
The exposing lamp irradiates a light beam onto a document to expose a photosensitive body by a reflected light beam to form an image. The exposing lamp is turned on and off each time a copy is made. The ON/OFF action of the exposing lamp is controlled, for example, by controlling the phase of an AC voltage from a power source to which the exposing lamp is connected. However, if the control device breaks and the exposing lamp stays on and continues to irradiate a light beam after the light-up period has passed, an excessive amount of light is irradiated onto the photosensitive body, thereby forming an unsatisfactory image. Also, the outer package of the copying machine may deform due to the heat generated by the exposing lamp, or at worst, smoking may occur from the outer package.
To prevent such incorrect light-up of the exposing lamp, for example, Japanese Laid-Open Patent Application No. 4-372942/1992 discloses a structure judging whether or not a copy lamp (exposing lamp) stays on erroneously based on a detection output of a document density sensor to stop the power supply to such an erroneously lit-up copy lamp.
An amount of light of the exposing lamp is adjusted, for example, by controlling the phase angle using a reference level signal generated based on a PWM (Pulse Width Modulation) signal from a CPU. However, if the CPU has a runaway or the like and continues to output the PWM signal, the reference level signal stays at the maximum level (the reference level is held at the maximum level) and hence the exposing lamp stays on. In such a case, the heat generated by the exposing lamp may deform the outer package, or cause smoke from the outer package at worst.
To prevent such continuous light-up of the exposing lamp caused by a runaway of the CPU, Japanese Laid-Open Patent Application No. 5-49158/1993, for example, discloses a structure, in which an ON state period of the PWM signal generated by the CPU is measured by a timer, and a time-up signal is outputted to a light-up circuit of the exposing lamp after a predetermined period to forcibly stop the power supply to the exposing lamp.
On the other hand, the fusing lamp raises the temperature of a heat roller provided in a fusing device. The temperature of the heat roller is stabilized at a predetermined level by, for example, turning on and off the fusing lamp, which is carried out based on a detection output of a temperature sensor provided near the heat roller.
The exposing lamp and fusing lamp are connected to the same power source in parallel and consume relatively a large amount of power compared with the other power consuming members in the electrophotographic apparatus. Thus, the power source voltage varies depending on the ON/OFF state of either the exposing lamp or fusing lamp, which changes the voltage applied to the other. More precisely, when the fusing lamp is turned on or off while the exposing lamp stays on, each ON/OFF switching action of the fusing lamp changes a voltage supplied to the exposing lamp, thereby making an amount of irradiation light uneven. Accordingly, as shown in FIG. 24, a resulting image has inconsistencies in monochromic density, namely, inconsistencies in copy density, in a portion copied at the moment the ON/OFF state of the fusing lamp is switched, thereby degrading the image quality.
To solve the above problem, Japanese Utility Model Gazette No. 58-53552(1983) discloses a temperature control device for the fusing lamp, which inhibits the fusing lamp's ON/OFF control while the exposing lamp is in the ON state (stays on), and allows the same while the exposing lamp is in the OFF state (goes off). According to this structure, variance in voltage caused by the fusing lamp's ON/OFF switching during the exposing process can be prevented, thereby making it possible to produce a satisfactory image without any inconsistency in copy density.
Incidentally, to realize satisfactory fusing, the temperature of the heat roller of the fusing device must be maintained in a predetermined range. Temperatures out of the predetermined range causes defective fusing (low temperature offset when the temperature is below the lower limit of the predetermined range, high temperature offset when the temperature is above the upper limit of the predetermined range). A thin heat roller of about 1 mm thick having a small heat capacity is used in general to facilitate the temperature control.
However, if the ON/OFF control of the fusing lamp is inhibited while the exposing lamp stays on as is disclosed in the above Japanese Utility Model Gazette No. 58-53552(1983), and in particular, if the heat roller provided in the exposing lamp is thin and has a small heat capacity, the temperature of the fusing device drops below the predetermined range (the range in which satisfactory fusing can be realized) when the fusing lamp is in the OFF state while the exposing lamp stays on, thereby possibly causing the low temperature offset. On the other hand, the temperature of the fusing device rises above the predetermined temperature range when the fusing lamp is in the ON state while the exposing lamp stays on, thereby possibly causing high temperature offset.
Therefore, although variance in voltage of the exposing lamp can be curbed and uneven exposure can be eliminated by inhibiting the fusing lamp's ON/OFF control while the exposing lamp stays on, there readily occurs defective fusing, such as low temperature offset or high temperature offset. Accordingly, the above structure presents a problem that the quality of a resulting image is degraded.